<?php echo get_post_meta($post->ID, 'image', true); ?>
Above code should work but most likely $post->ID would need to be switched to another array, post_parent
see ref:
https://codex.www.ads-software.com/Class_Reference/WP_Query
https://codex.www.ads-software.com/Class_Reference/WP_Query#Post_.26_Page_Parameters
https://developer.www.ads-software.com/reference/functions/get_the_post_thumbnail/